Aquatic dead zones occur when the dissolved oxygen levels (typically below 2 mg/l) in a body of water are so low that marine life is unable to survive. A biological desert, one cause of hypoxia is runoff from nitrogen and phosphorus-based fertilizers. These excess nutrients from agricultural operations are washed into rivers, lakes, and seawater during periods of rainfall. The introduction of excess nitrogen and phosphorous then stimulates algae growth. As algae blooms die off and settle at the body of the water, the decomposition of algae by bacteria results in a depletion of oxygen, triggering the hypoxic condition.
A newly released forecast by the National Oceanic and Atmospheric Administration (NOAA) has predicted that the Gulf of Mexico will again become the second-largest low-oxygen dead zone on after (after the Baltic Sea). Each year, agricultural runoff triggers a dead zone along the bottom waters of the northern Gulf of Mexico continental shelf. Researchers have analyzed the discharge rates and nitrogen loads of the Mississippi and Atchafalaya Rivers to predict that 22,557 km2 (8,717 mi2) of the bottom of the continental shelf off Louisiana and Texas will become a dead zone during late July of 2019.
Researchers have been systematically mapping the hypoxic zones of the northern region of Gulf of Mexico since 1985. Between 1985 to 2018, the geographic range of the dead zone that appears in the summer has ranged between 40 to 22,720 km2 during July and averaged 14,042 km2 (5,424 mi2).
- Turner, R. E., & Rabalais, N. N. (2019). 2019 Forecast: Summer Hypoxic Zone Size Northern Gulf of Mexico. Louisiana State University, Website PDF article.
- Gulf Dead Zone Looms Large in 2019, EOS